Glutathione synthetase: similarities of the proteins from Schizosaccharomyces pombe and Arabidopsis thaliana

Glutathione synthetase predicted from the reported gene sequence from Schizosaccharomyces pombe is substantially smaller than the equivalent protein predicted from the cDNAs sequenced fromArabidopsis thaliana, Saccharomyces cerevisiae and other eukaryotes. Sequence alignments of the proteins encoded by the cDNA clones for glutathione synthetase from Arabidopsis and S. pombe show that the Arabidopsis protein contains 200 extra amino acids at the N-terminus. In order to test if this sequence is essential in the function of the protein, the full-length Arabidopsis protein and as two N-terminal deletions (Δ67–71 and Δ67–200) were expressed in S. pombemutant MN101, which lacks endogenous glutathione synthetase activity. Although the wild-type plant cDNA could complement the yeast mutation, neither deletion mutant was able to restore glutathione-dependent cadmium resistance. When the three proteins were expressed as fusion proteins in Escherichia coli, they accumulated to the same level, but only the plasmid containing the full-length cDNA, pFLAG222, produced detectable enzyme activity in vitro. These results suggested that the N-terminus of the Arabidopsisglutathione synthetase is essential for its function and opened up the possibility that there was a sequencing error in the reported S. pombe sequence. Therefore the gsh2 sequence from wild-type S. pombeand the mutant strain MN101 were determined. The wild-type S. pombe gsh2 encodes a protein that is about the same length as that found in Arabidopsis, and the MN101 mutation involves a frameshift mutation early in the glutathione synthetase reading frame.